1. Field of the Invention
The present invention relates to injectors and nozzles for gas turbine engines, and more particularly, to mechanisms for purging fuel within a fuel injectors for gas turbine engines.
2. Description of Related Art
A variety of devices and methods are known in the art for injecting fuel into gas turbine engines. Of such devices, many are directed to injecting fuel into combustors of gas turbine engines under high temperature conditions. For example, fuel injectors for gas turbine engines on an aircraft direct fuel from a manifold to a combustion chamber of a combustor. This fuel injector typically has an inlet fitting connected to the manifold for receiving the fuel, a fuel nozzle located within the combustor for spraying fuel into the combustion chamber, and a housing stem extending between and fluidly interconnecting the inlet fitting and the fuel nozzle. The housing stem typically has a mounting flange for attachment to the casing of the combustor.
Fuel injectors are typically heat-shielded because of high operating temperatures arising from high temperature gas turbine compressor discharge air flowing around the housing stem and nozzle. The heat shielding prevents the fuel passing through the injector from breaking down into its constituent components (i.e., “coking”), which may occur when the wetted wall temperatures of a fuel passage exceed 400° F. Coking in the fuel passages of a fuel injector can build up to restrict fuel flow to the nozzle.
Typically, conventional injectors include annular stagnant air gaps as insulation between external walls, such as those in thermal contact with high temperature ambient conditions, and internal walls in thermal contact with the fuel. In order to accommodate differential expansion of the internal and external walls while minimizing thermally induced stresses, the walls are anchored at one end and free at the other end for relative movement. If the downstream tip ends of the walls are left free for relative movement, even a close fitting sliding interface between the downstream tip ends can allow fuel to pass into the air gap formed between the walls. For example, fuel can be drawn into these air gaps due to a capillary effect due to changing pressures when the engine is shut down. Ultimately, this can result in fuel being stored in the air gaps. When the fuel becomes sufficiently heated, the fuel can break down and form carbon in the air gap, which carbon is not as good of an insulator as air, or, prior to breaking down, the fuel can even combust. Such combustion can ultimately damage the fuel injector and, in extreme cases, can damage the entire engine causing a failure. In addition, the carbon may build up to a point where it blocks venting of the air gap to the stem, which can lead to diminished injector service life and may require frequent and costly cleaning of the fuel injector.
Such conventional methods and systems generally have been considered satisfactory for their intended purpose. However, there still remains a continued need in the art for nozzles and fuel injectors that properly insulate while reducing or preventing fuel entry (and thus carbon entry) in the insulation gaps. The present invention provides a solution for these problems.